During the drilling of an oil or gas well successive measurements are usually made of various borehole conditions as well as one or more properties of the earth formations that are being penetrated by a drill bit as it progressively excavates the borehole. Even though it was recognized that drilling operations would be greatly improved if measurements such as these could be frequently made, in the past many of these measurements simply could not be obtained unless the drill string and drill bit were temporarily removed from the borehole and one or more wireline logging operations were conducted in the open borehold. Since wireline logging operations can significantly prolong the time needed to complete a borehole, heretofore the usual practice has been to minimize as far as possible the number of so-called "open hole logs" that are run during the drilling of a given borehole.
Those skilled in the art will, of course, recall that different proposals have been advanced heretofore for making one or more of these measurements without removing the drill string and bit. For instance, as depicted in U.S. Pat. No. 3,112,442, it was proposed to provide a self-contained instrument including a suitable power supply, a recorder and various electrical and/or radioactivity sensors that was adapted to be moved through the longitudinal bore of a drill string and landed on a suitable seat just above the drill bit. Once a series of measurements were taken, a so-called "wireline overshot" was lowered through the drill string and coupled to a fishing neck on the upper end of the instrument housing for returning the entire instrument to the surface for analysis of the measurements. A similar arrangement is shown in U.S. Pat. No. 3,209,323 which instead transmitted the recorded measurements to the surface by way of a typical logging cable carrying an overshot having a winding that was inductively coupled to a matched winding in a fishing neck on the instrument. As seen in U.S. Pat. No, 3,186,222, another prior-art proposal employed a self-contained measuring assembly having electrical and/or radioactivity sensors that was mounted on the lower end of the drill string just above the bit. With this arrangement, the output signals from the measuring assembly were converted into successive alternately-polarized electromagnetic pulses which were transmitted along the walls of the drill string to surface detectors by means of self-contained repeater stations tandemly coupled at spaced intervals in the drill string. Since the radioactivity logging devices disclosed in the three above-cited patents were designed to measure only the natural gamma radiation from the earth formations, those logging devices did not require a source of radioactivity.
It will, of course, be appreciated that many of the problems experienced by these several prior-art systems were effectively resolved by the advent of various measuring-while-drilling or so-called "MWD" tools. With the introduction of the MWD tools that are now commercially available, for the first time it became practical to transmit to the surface one or more real-time downhole measurements without interrupting the drilling of a borehole. As described, for example, in greater detail in U.S. Pat. No. 3,855,857, a typical commercial MWD tool may measure such downhole conditions as the so-called weight-on-bit or "WOB" as well as the torque acting on the bit, the azimuthal direction and the angle of inclination of the borehole, borehole pressure and temperature, mud resistivity and various characteristics of the earth formations penetrated by the bit. The output signals of the various sensors are coupled to circuits which selectively control a downhole acoustic signaler in the tool for successively transmitting encoded data signals representative of these real-time measurements through the mud stream in the drill string to suitable detecting-and-recording apparatus at the surface.
It will, of course, be appreciated that MWD tools have been proposed heretofore for providing real-time measurements of different radioactivity characteristics of the earth formations being penetrated by the drill bit. Since measurement of natural gamma radiation requires only a gamma-ray detector and typical circuits to control the signaler, it has not been difficult to provide MWD tools with that instrumentation. Typical MWD tools with this capability are shown, for example, in U.S. Pat. No. 4,520,468 as well as in FIG. 4 of U.S. Pat. No. 3,255,353. On the other hand, as depicted in FIG. 1 of the last-cited patent, to measure other radioactivity characteristics of earth formations, a MWD tool must also have an appropriate source of radiation such as a radioactive chemical source. Since the measurement of formation density is impaired by borehole fluids, as seen, for example, in U.S. Pat. No. 4,596,926 it has been proposed to compensate for the effect of the fluids by arranging an array of radioactive sources and radiation detectors around the tool body.
Those skilled in the art recognize that any time a MWD tool is being used in a drilling operation, it is always possible that the tool may be inadvertently stuck in the borehole. Should the tool or drill string become firmly stuck, it may be necessary to retrieve the upper portion of the drill string and then employ one or more "fishing" techniques to recover the remaining portion of the drill string and the MWD tool from the borehole before the drilling operation can be resumed. Such fishing operations may, however, impose such severe impacts on a MWD tool that its inner components will be severely damaged before the tool is recovered. Thus, should a MWD tool carrying a chemical radioactive source become stuck to such an extent that the tool cannot be readily recovered, it is possible that the radiation shield around the source can be ruptured. If this occurs, the borehole fluids will be contaminated and it may be risky to handle the damaged tool when it is recovered at the surface. Accordingly, the potential risk must always be considered when the conditions in any given borehole are such that the MWD tool may become stuck. In some situations it may be doubtful that the advantages of using a tool with a chemical source justify using that tool while drilling a borehole interval in which the tool may be stuck. As a result, not only will the service company running the MWD tool lose the additional revenue that it would otherwise receive, but the well operator will also forego the data that would have been provided by those measurements dependent upon a radioactive source.
To overcome such problems, various proposals have been made heretofore to provide self-contained instruments that can be moved through the drill string and temporarily stationed in one of the drill collars just above the drill bit. For example, as described in U.S. Pat. No. 4,041,780, a self-contained logging instrument is arranged to be pumped through the drill string to a landing seat that has been temporarily installed on the lower end of the drill string. It is, of course, readily apparent that the major disadvantage of this instrument is that the drill bit must be temporarily replaced with the sub carrying the seat every time a series of measurements are to be made. Thus, since each series of measurements requires two complete round trips of the entire drill string, ordinarily it will be far more practical to simply employ a wireline logging tool for obtaining these measurements while the drill string is out of the borehole. U.S. Pat. No. 4,550,392 also describes a similar self-contained instrument that is moved into and out of the drill string by a cable. However, even though this instrument is installed and removed while the drill string and drill bit are in the borehole, since the sensors in this instrument are always within a thick-walled drill string, some formation radioactivity characteristics simply can not be effectively measured. Moreover, once that instrument has been removed from the drill string, the drilling operation must be continued without the benefit of further downhole measurements.